an engineering geological characterisation of tropical clays - GBV

1
Chapter 1
Introduction
1.1 Scope of study
The purpose of this project was to study and evaluate engineering geological characteristics of soils
found in the Nairobi area, Kenya. The information and data so obtained would be useful for
assessing the suitability and capacity of soils found regarding the possibilities and / or limitations of
developing and constructing low buildings and other light engineering structures ( roads, pavements
etc) in the study area and geologically similar neighbourhoods. Attempts have been made by the
author to present the geological data and soil parameters in a spatial continuous form that would be
readily useful and applicable by engineering geologists, geo-engineers and planners of futureintended
construction projects in the area. The results of the investigations together with related
engineering geological assessments would assist especially at the stages of planning, designing and
construction of projected facilities; and also subsequent maintenance after completion.
The current study area lies mainly to the south of the fast growing Nairobi city, Kenya. However,
the southern half of the Nairobi metropolitan, including the City centre, is also covered. The area is
covered by topographic sheet 148/4 of Nairobi, at a scale of 1:50 000; and printed by the Survey of
Kenya (1991). The exact location and extent of the study area is described in the next section. The
area is covered mainly by varying thickness of black cotton soils. Occurrences of red soils are also
found, but limited in the north-western part of the area. Underlying the soils are Tertiary volcanics
which in turn overly a Basement system of folded Precambrian metamorphic rocks.
The present area was chosen because it is mainly located on the outskirts of Nairobi city which are
projected for future expansion of the town, as regards construction of new social-economic and
industrial facilities, residential houses and related infrastructure. It is also largely covered by the
potentially expansive and reactive, relatively unstable and problematic black cotton soils. Previous
works and studies in adjacent areas and sections of the present area (Anyumba, 1991; Manga,
1988; Mowlem, 1985), together with recent field observations by the author reveal problems of
ground instabilities, failures and movements in areas of black cotton soils with serious engineering,
environmental and social-economic implications.
The field study phase of this project was carried out in Nairobi (Kenya) in the period of April-
September, 1999; and December, 2000-January, 2001. The laboratory study phase was undertaken
in the period of November, 1999-March, 2001; based at the Department of Engineering Geology,
Institute of Geology and Palaeontology, Technical University of Clausthal-Germany. The field
methods and laboratory procedures followed in the investigation, study and analysis of the soils and
underlying bedrock are summarised in the relevant sections in later chapters.
Adequate and proper engineering geological studies and investigations should serve to provide
useful and detailed information on aspects of geology (rock types and lithostratigraphy), types of
soil, geomorphology and geological structures, hydrology and hydrogeology, and geotechnical
characteristics and potential behaviour of rocks and/ or soils; followed by an assessment of potential
instabilities and possible natural hazards, and a subsequent consideration of suitable development
projects (Attewell, 1976; Bell and Pettinga, 1985; Blyth, 1974; Hedberg 1976; Sowers, 1994;
Tilford, 1994). In addition to the above investigations, this study also includes a detailed chemical
and mineralogical analysis and evaluation of the soils of the area. Genetic aspects of the soils are
also discussed.